In general, a floating support has anchor means for keeping it in position in spite of the effects of currents, winds, and swell. It also generally comprises means for storing and processing oil and means for transferring it to offloading oil tankers, which call at regular intervals to offload production. Such floating supports are referred to below by the acronym “FPSO” which stands for floating production storage offloading.
Because of the multiplicity of lines that exist in that type of installation, it has been necessary to implement bottom-to-surface connections of the hybrid tower type in which substantially vertical rigid pipes referred to herein as “vertical risers” provide connections between undersea pipes resting on the sea bottom and rise up a tower to a depth that is close to the surface, and from this depth flexible pipes provide connections between the tops of the towers, i.e. the tops of the vertical risers, and the floating support. The tower is then provided with buoyancy means so as to remain in a vertical position and the risers are connected at the foot of the tower to undersea pipes via rigid sleeves for absorbing the vertical movements of the tower. Overall the assembly is commonly referred to as a “hybrid tower” since two technologies are involved, firstly a vertical portion or tower proper in which the riser is constituted by rigid pipes, and secondly the top portion of the riser which is constituted by pipes in a catenary configuration for providing a connection with the floating support.
French patent No. FR 2 507 672 published on Dec. 17, 1982 and entitled “Colonne montante pour les grandes profondeurs d'eau” [A riser for great depths] describes one such hybrid tower.
The present invention relates more particularly to the known field of connections of the type comprising a vertical hybrid tower anchored to the sea bottom and comprising a float situated at the top of a vertical riser, which is in turn connected via a pipe and in particular a flexible pipe that takes up a catenary configuration under the effect of-its own weight on going from the top of the riser to a floating support on the surface.
The advantage of such a hybrid tower lies in the ability of the floating support to depart from its nominal position while inducing minimal stresses on the tower or on those portions of the pipe that take up a suspended catenary configuration, whether underwater or on the surface.
Patent publication WO 00/49267 in the name of the present Applicant discloses a tower whose float is at a depth that is more than half the depth of the water and in which the catenary connection to the surface vessel is in the form of thick-walled rigid pipes. At its base, the tower described in that document requires flexible connection sleeves enabling the bottom ends of the vertical risers of said tower to be connected to the undersea pipes resting on the bottom in order to absorb the movements that result from expansion due to the temperature of the fluid being transported.
More particularly, in WO 00/49267, the anchor system comprises a vertical tendon constituted either by a cable or by a metal bar or indeed by a pipe held under tension at its top end by a float. The bottom end of the tendon is fixed to a base resting on the bottom. Said tendon has guide means distributed all along its length, and through which said vertical risers pass. Said base can merely be placed on the sea bottom and can remain in place under its own weight, or it can be anchored by means of piles or any other device suitable for holding it in place. In WO 00/49267, the bottom end of the vertical riser is suitable for being connected to the end of a curved sleeve that can be moved between a high position and a low position relative to said base, from which said sleeve is suspended and associated by return means urging it towards the high position in the absence of a riser. This mobility of the curved sleeve serves to absorb variations in riser length due to the effects of temperature and pressure. At the head of the vertical riser, an abutment device secured thereto bears against the support guide installed at the head of the float, thereby holding the entire riser in suspension.
In addition, since the crude oil is conveyed over very long distances, e.g. several kilometers, it is desirable to provide an extreme degree of insulation both to reduce any increase in viscosity which would lead to a reduction in the hourly production rate of a well, and secondly to avoid any blockage of the flow by deposition of paraffin or by the formation of hydrates once the temperature drops to around 30° C.-40° C. These phenomena are particularly troublesome in West Africa where the temperature of the sea bottom is about 4° C. and where crude oils are of the paraffin type.
Numerous thermal insulation systems are known which enable the required level of performance to be achieved while withstanding pressure at the bottom of the sea which can be about 150 atmospheres at a depth of 1500 meters (m). Amongst the various concepts available, mention can be made of the “pipe-in-pipe” system comprising a pipe conveying the hot fluid which is installed inside an outer protective pipe, with the space between the two pipes being either merely filled with an insulating substance which is optionally vacuum confined, or else the space can merely be evacuated. Numerous other kinds of material have been developed for providing high temperature insulation, some of which are also capable of withstanding high pressure, merely surrounding the hot pipes and generally being confined within a flexible or rigid outer casing, itself in pressure equilibrium and having the main function of ensuring that the geometrical shape of the material remains substantially constant over time.
All of those devices for conveying a hot fluid within an insulated pipe present differential expansion phenomena to some extent. The inner pipe is generally made of steel and is at a temperature which it is desired to keep as high as possible, e.g. 60° C. or 80° C., while the outer casing, often likewise made of steel, is at the temperature of sea water, i.e. around 4° C. The forces generated on the elements providing interconnection between the inner pipe and the outer casing are considerable and can reach several tens or even several hundreds of (metric) tonnes, and the resulting total elongation is about 1 m to 2 m for insulated pipes that are 1000 m to 1200 m in length.